A gas turbine engine includes an engine outer structure, which is essentially a pressure vessel that contains hot, flowing air, and support structures for the rotating elements of the engine, which extend through the pressure vessel in order to support the rotating elements whilst still allowing air to pass from the front to rear of the engine.
Generally, the support structures are circular when viewed along the axis of the engine, with a number of struts or vanes joining inner and outer rings and a bearing chamber located in the middle. Inside the bearing housings, the bearings allow free rotation, yet precise centring, of the rotating elements of the engine such as drive shafts.
The bearing chamber provides a favourable environment for the bearings to operate reliably. For example, inside the chamber, nozzles are provided to distribute lubricant to the bearings. To prevent lubricant leaking out of the bearing chamber, and to prevent the ingress to the chamber of excess hot air, the bearing chamber is separated from the surrounding environment by seals. In use, the air pressure in the environment surrounding the bearing chamber is at a higher pressure than inside the chamber, and typically gas flows in to the chamber through the seals. Thus the seals may be provided in the form of labyrinth seals, for example. This inward flow of air prevents oil from migrating in the opposite direction out of the bearing chamber through the seal.
However, this arrangement can be disadvantageous. For example, the inward flow of gas may bring with it unwanted detritus, debris and/or other undesirable contaminants, which could adversely affect the operation of the bearings.
Furthermore, the incoming gas may be of a sufficiently high temperature to create an axial thermal gradient across the bearing. This may cause the bearing cage to warp, e.g. to cone, and may result in dynamic cage instability of the bearing.
It may also cause a temperature rise in the lubricant (oil) in the bearing, potentially leading to degradation of the lubricant and thus to bearing damage. Indeed, lubricant is typically scavenged from the bearing and reused elsewhere in the engine, thus degraded lubricant could have adverse effects elsewhere in the engine.
The architecture of the engine will often dictate when a bearing and a seal are in axial alignment, or are close to axial alignment. In such an alignment, the typical route of the gas flow from the seal is against the direction of lubricant flowing from the bearing, and this impact of lubricant (in particular, oil) and gas (in particular, air) may cause local disruption to the oil/air flow and increase the temperature of the oil in the bearing chamber, leading to similar disadvantages as those described above.